There are different methods or systems for producing oriented pipes, which can be grouped into two broad categories: Continuous or in-line systems and discontinuous or in-batch systems.
Discontinuous or in-batch systems are mostly based on producing the molecular orientation “item by item”, which is achieved by expanding the blank in a mould that provides the final shape of the tubular profile.
In-line processes are radically different and the orientation takes place continuously and simultaneously to the extrusion itself, there being no interruption in the process, or intermediate stock, etc.
Returning to the in-batch molecular orientation process, the blank pipe is placed inside a mould and is expanded against it by means of internal pressure, it being a fundamental part of the process to secure the pipe in a plastic state that is to be expanded. The internal pressure that is generated inside the pipe causes an axial stress that separates the plastic pipe from its grips. Depending on the working conditions, these stresses can be high and abrupt. Bearing in mind that the pipe is in a plastic state, i.e. with an elastomer-like appearance and easily malleable, it is difficult to ensure that it is reliably secured, as the grips themselves deform the pipe that is being secured and it may slide out of them. Furthermore, as it is soft, this plastic is particularly sensitive to tearing, producing shavings, etc., and if this happened, it could eventually jam the system and render the mechanism for continuous use inoperative. Additionally, for the pipe that is to be oriented to be in a plastic state it must be hot, at temperatures above 100° C., which makes it difficult to use grips made from materials other than metals, such as polymers.
Numerous patent documents could be cited that use ball systems to lock two solid pieces together, or patents that explain methods of securing pipes to their moulds.
The first group includes ES 131894 and JP 11210969. Both patents disclose a fast connection system for pipes, with the characteristic that in both cases the pieces to be gripped are solid, i.e. they cannot be deformed by the action of the connection mechanism. Furthermore, the balls only act as a blocking element of the pieces and not as a pressure element.
Other documents are cited below, the concept of which is to lock the pieces:    DE 3518506: “Low pressure closure for gaseous and fluid media, having a ball/detent connection”    ES 2182979 “Quick connection method and device, and surgical instrument for driving interchangeable rotary tools”    ES 2576637: “Quick connection device for tubular bodies”
The following examples of systems for manufacturing oriented pipes with moulds could be cited:    WO9856567: “Process for fabricating an axially oriented plastic pipe thus obtained”    U.S. Pat. No. 4,340,344: “Apparatus for manufacture of thermoplastics pipes”    U.S. Pat. No. 4,428,900: “Method of forming a pipe of oriented thermoplastic polymer”    EPO 0072064: “Method and apparatus for manufacturing molecular oriented tubes”
All these patents use rudimentary connection systems, by simple clamping of two segmented semi-circumferential pieces or by simple pressure. This is because in these devices the pipe to be oriented is gripped in a cold, solid state, being heated in the mould itself. The concept of inserting a heated pipe for molecular orientation is a concept disclosed in patent applications filed by the same applicant as the present invention.
There are also several solutions for making permanent deformations on already fabricated and oriented pipes or tubes. Amongst said solutions, it's worth mentioning:    U.S. Pat. No. 3,640,115: “Tube spacer tool”    EP0611614: “Device for forming a mechanical connection between two concentric tubes”    U.S. Pat. No. 4,512,178: “Tube and deforming tool”
As earlier stated, the above mentioned documents describe solutions for forming permanent deformations in plastic pipes.
Furthermore, the German patent application DE3428191: “Process for producing a pressure-resistant packaging container” describes gripping and releasing tools configurations which are not suitable to be used in a mould for expanding pipes.